J. Phys. Chem. C 2007, 111, 901-907
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One-Dimensional Array of Au Nanoparticles Fixed on Nanofibers of Organogelators by the Langmuir-Blodgett Method Ryo Tsunashima,† Shin-ichiro Noro,†,‡ Tomoyuki Akutagawa,†,‡,§ Takayoshi Nakamura,*,†,‡,§ Tomohiro Karasawa,| Hiroko Kawakami,| and Kazunori Toma| Graduate School of EnVironmental Earth Science, Hokkaido UniVersity, Sapporo 060-0810, Japan, Research Institute for Electronic Science, Hokkaido UniVersity, Sapporo 060-0812, Japan, CREST, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan, and The Noguchi Institute, Tokyo 173-0003, Japan ReceiVed: August 7, 2006; In Final Form: October 31, 2006
The Langmuir-Blodgett (LB) method was applied to low molecular weight organogelator N-[N-(6hydroxyhexyl)-3-carbamoylpropyl]-3,4,5-tris(dodecyloxy)benzamide (1) to form molecular-assembly fibers on a solid substrate. Molecule 1 formed a stable monolayer up to 40 mN/m at the air-water interface. The monolayer was transferred onto a mica surface at 35 mN/m. In the transferred film, fibrous structures with typical dimensions of 1.2 nm (height) × 50-100 nm (width) × 5-10 µm (length) were observed by atomic force microscopy. These structures were ordered in specific directions, reflecting the C6 surface crystal symmetry of mica. Lateral modulation frictional force microscope measurements revealed that the fibrous structures had high crystallinity and that the surface groups were hydrophilic. These results enabled us to construct functional fibers by introducing functional units into the hydrophilic part of molecule 1. N-[N-(6-Aminohexyl)3-carbamoylpropyl]-3,4,5-tris(dodecyloxy)benzamide hydrochloride (2) has an ammonium moiety instead of the terminal alcohol moiety of 1. By adding 2 to 1, we introduced adsorption sites for Au nanoparticles on the surface of the fibrous structure. By casting a Au nanoparticle suspension onto this fibrous structure, we successfully constructed Au nanoparticle arrays in which Au nanoparticles were adsorbed on a mixed fibrous structure of 1 and 2 on mica substrates.
Introduction Recently, nanomaterials have been attracting much interest as building blocks of electronic devices.1-3 So far, nanoscale electronic circuits have been produced mainly by electron-beam lithography and other dry fabrication techniques. However, dry fabrication processes become extremely costly with increasing degree of integration. Bottom-up self-assembly techniques will provide us with economical fabrication methods for nanoscale integration circuits from nanoscale building blocks. Nanomaterials exhibit novel phenomena that are not observed in the bulk state, such as quantized conductance, Coulomb blockade, and metal insulator transition. Nanowires,4-8 nanotubes,9,10 and nanoribbons11,12 are typical one-dimensional nanostructures. They play an important role in connecting electrically active functional units in nanoscale electronic circuits, and their quantum transport properties at the nanoscale are extensively studied.1-4,7 As a zero-dimensional nanoscale building block, Au nanoparticles13 are promising. They are stable and easy to synthesize and handle, and their diameter is controllable from 1 nm, at which they show quantum properties, to over 100 nm, at which they show bulk properties.13-15 For example, in Au nanoparticle arrays, electron transfer between nanoparticles is governed by the tunneling through the barrier of insulating ligand shell covering the Au nanoparticles.16,17 These properties were applied to single electron tunneling devices.14 In these * Corresponding author: tel +81-11-706-2849; fax +81-11-706-4972; e-mail
[email protected]. † Graduate School of Environmental Earth Science, Hokkaido University. ‡ Research Institute for Electronic Science, Hokkaido University. § CREST, Japan Science and Technology Agency. | The Noguchi Institute.
devices, Au nanoparticles were randomly distributed. By using a regular array of Au nanoparticles, devices with much higher functions can be obtained. However, fabrication techniques of Au nanoparticles through a bottom-up self-assembly process have not yet been established. Low molecular weight organogelators possess a threedimensional network structure in a gel. They assemble in quasione-dimensional fibers that are entangled with each other. The fibrous molecular assemblies are typically constructed through strong anisotropic intermolecular hydrogen bonding. Wet techniques such as cast- and spin-coating and Langmuir-Blodgett (LB) methods give ultrathin films of nanoscale thickness.18,19 The LB technique, in particular, provides a way to control the film thickness through layer-by-layer deposition. In an LB layer, the orientation of molecules is ordered due to the amphiphilic character of the materials. In addition, LB layers often showed nanostructures such as domains, dots, and wires due to anisotropic intermolecular interactions.5,6,20 Thus, application of the LB method to organogelators can produce low-dimensional nanostructures on solid substrates, like the fibrous network structures seen in organogels.6,21-24 In addition, the introduction of adsorption sites for Au nanoparticles such as ammonium moieties on the surface of the fibrous structure of the organogelator enables us to obtain a low-dimensional array of Au nanoparticles.25 Here, we employed N-[N-(6-hydroxyhexyl)-3carbamoylpropyl]-3,4,5-tris(dodecyloxy)benzamide (1, Chart 1 ) as a low molecular weight organogelator. Molecule 1 has three hydrophobic alkoxyl chains at the 3-, 4-, and 5- positions of benzamide, while an alcohol moiety is introduced at the opposite end of the molecule as a hydrophilic moiety. We found that the molecular assembly of 1 fabricated on the mica substrate
10.1021/jp065061j CCC: $37.00 © 2007 American Chemical Society Published on Web 12/07/2006
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CHART 1: Molecular Structures of Organogelators 1 and 2
by use of the LB technique showed a fibrous structure. N-[N(6-Aminohexyl)-3-carbamoylpropyl]-3,4,5-tris(dodecyloxy)benzamide hydrochloride (2) which has an ammonium moiety at the end instead of the hydroxyl group of 1, was introduced to form a mixed fibrous structure with 1. Au nanoparticles were successfully absorbed on the mixed fibrous structure on the substrate, forming a one-dimensional array. Experimental Methods Materials. The synthesis of 1 and 2 was carried out in a similar fashion to other 3,4,5-tris(alkyloxy)benzamide derivatives26 and is described in the Supporting Information. The gel state was confirmed by the general method.23 The compounds were dissolved in hot organic solvents. When these solutions were slowly cooled down to room temperature, the formation of jellylike solid was observed. The suspension of Au nanoparticles was prepared according to a method reported previously and the diameter of Au nanoparticles was confirmed by scanning electron microscopic measurement (SEM) and UV-vis spectra.27 Film Preparation. A conventional Langmuir trough (NIMA 5152D) was used for Langmuir film formation and LB film deposition. Chloroform-methanol (9:1) solutions of compounds 1, 2, and a mixture of these two (0.5 mM) were spread on a pure water subphase. The π-A isotherms at 291 K were recorded at a barrier speed of 100 mm/min. Films were deposited at a surface pressure of 20, 25, 35, or 45 mN/m by a single upstroke withdrawal of a mica substrate at 5 mm/min (vertical dipping method). For the absorption of Au nanoparticles, 50 µL of a 0.5 mM Au nanoparticle suspension was cast on an LB film. After 30 min, the droplet was removed by slanting the substrate. Measurements. Atomic force microscope (AFM) and lateral modulation-frictional force microscope (LM-FFM) images were taken at ambient conditions by a SPA-400 multifunction unit with an SPI 3800 probe station (SII NanoTechnology Inc.) or a JSPM-5200 environmental scanning microscope (JEOL Inc.) operating in dynamic force mode (for topographies) and contact mode (for frictional images), with commercially available silicon nitride cantilevers having spring constants of 15 and 0.08 N/m for AFM and LM-FFM, respectively. LM-FFM measurements were obtained in conventional fashion by scanning the cantilever perpendicular to its long axis at constant load applied with a vibration of 10.0 nm amplitude and 3.0 kHz frequency. Results and Discussion Fabrication of LB Films of Organogelator 1. Figure 1 shows (a) the surface pressure-area (π-A) and (b) compress-
Figure 1. (a) Surface pressure-area (π-A) and (b) compressibilityarea (C-A, blue line) and π-A (black line) isotherms of molecule 1.
ibility-area (C-A) isotherms of 1. The compressibility was calculated from the π-A isotherm and is defined as
C)-
1 ∂A A ∂π
( )
The surface pressure began to increase at around A ) 1.2 nm2, and the monolayer was stable up to 40 mN/m. A shoulder plateau was observed between 0.70 and 0.85 nm2, with a surface pressure of 8 mN/m. Upon further compression, the compressibility remained almost constant from 25 to 40 mN/m, and the Langmuir film showed a solid condensed phase whose extrapolated area at 0 mN/m (0.60 nm2) was comparable to the crosssectional area of three alkyl chains, showing that alkyl chains were closely packed at the air-water interface. Compressibility increased above 40 mN/m, showing collapse of the Langmuir film at the air-water interface. LB films of 1 were deposited on a mica surface at 20, 25, 35, and 45 mN/m by a single withdrawal (Figure 1b, locations a-d). Figure 2 shows AFM images of LB films of 1 deposited at 20, 25, 35, and 45 mN/m (panels a-d, respectively). The transfer ratios of these films were close to unity, and the coverage of these films on mica calculated from AFM images were 80-90% except for the film deposited at 45 mN/m. In the AFM image of the film deposited at 35 mN/m, fibrous structures (black-edged white arrow in Figure 2c) similar to those seen in gels were observed. The fibrous structures were 1.1-1.2 nm in height, 50-100 nm in width, and 5-10 µm in length. In addition, two types of other structures were observed, namely, (i) a uniaxially oriented molecular assembly that formed oval domains with the principal axis aligned in the direction indicated by the black arrows in Figure 2a-c and (ii) a uniform domain that did not form specific nanostructures, indicated by a gray arrow in Figure 2c. The fibrous structures and orientation axes of the uniaxial molecular assemblies were oriented in specific directions, forming a 60° or 120° angle with respect to each other, which reflects the C6 crystal symmetry of the mica surface. The fibrous structures oriented along the crystal lattice direction of mica, possibly caused by the electrostatic interaction between the periodic array of atoms (ions) of mica and the
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Figure 2. AFM images of LB films on a mica substrate formed by a single withdrawal at (a) 20, (b) 25, (c) 35, and (d) 45 mN/m. The black, black-edged white, and gray arrows in the height profile correspond to uniaxial molecular assembly, fibrous structure, and uniform domain, respectively. The height profile of Figure 2d corresponds to the boxed area of the inset.
molecular assemblies.11,28 Similar fibrous structures were observed on the aminosilylated silicon substrate, although the direction of fibrous structures was random (Figure S1, Supporting Information). The thickness of the uniaxial molecular assembly (0.9-1.0 nm) was slightly larger than that of the uniform domain (0.6-0.8 nm). The height of the three structures increased in the order of uniform domain < uniaxial molecular assembly < fibrous structure, as seen in the height profile. This is presumably related to the difference in molecular orientation and/or bilayer formation because the height of the fibrous structure was almost twice that of a uniform domain. Only uniaxial molecular assemblies were observed in the case of the film deposited at 20 mN/m (Figure 2a). The molecular assemblies constituted the major fraction of the film deposited at 25 mN/m as well, but the latter also exhibited fibrous structures,
although the height was almost the same as the uniaxial molecular assemblies. These results suggest that fibrous structures were formed and grew by compression of the monolayer at the air-water interface. At 45 mN/m, the monolayer was collapsed, as also suggested by the π-A and C-A isotherms. In the AFM image (Figure 2d), fibrous structures were observed as in the case of gels. The surface morphology of cast film (xerogel) on mica surface was constructed from thick entangled fibrous structures (Figure S2, Supporting Information). Therefore, LB techniques are effective to construct oriented thin fiber structures. Molecule 1 formed nanofiber structures in addition to uniaxial molecular assemblies and uniform domains upon application of the LB method on the mica surface (Figure 2c). The surface of each domain was characterized by surface frictional force
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Figure 3. (a) Topographic and (b) frictional images of the LB film transferred at 35 mN/m.
Figure 4. Frictional signals as a function of the angle between the scanning direction and the oriented directions of the long axis of the fibrous structure (red squares) or the uniaxial molecular assembly (blue triangles).
Figure 5. Schematic view of the uniaxial molecular assembly and the fibrous structure.
measurements by use of LM-FFM. LM-FFM detects the torsion of a cantilever arising from the frictional force between a surface and the cantilever. The magnitude of the frictional force depends on the surface functional groups, molecular orientation, and crystallinity.29-33 Stronger interactions between the tip and surface give higher frictional forces. Figure 3 shows an LM-FFM image (together with the AFM image taken simultaneously) of the LB film transferred at 35 mN/m. The frictional force of the fibrous structure and uniaxial molecular assembly showed anisotropic properties. Figure 4 plots the frictional signals from a fibrous structure and uniaxial molecular assembly as a function of the angle between the scanning direction and the long axis of the fiber or uniaxial molecular assembly. The frictional force on mica was used as an internal standard and defined as 0 mV. Since K+ are oriented and ordered with C6 crystal symmetry on a cleaved mica surface, the frictional force observed is consistent everywhere and the spherical shape of K+ shows an isotropic frictional character.34 The minimum frictional force of these two domains was observed along the scanning direction parallel to the long axis. The maximum frictional force was observed along the perpendicular direction. Frictional anisotropy has been observed for materials with high crystallinity and tilt orientation of molecules such as long alkyl chains at the surface.31-33 Thus, molecule 1 was shown to tilt at the substrate surface, and the fibrous structure and uniaxial molecular assembly to have crystalline structures. The uniform domain showed no frictional anisotropy, suggesting that the molecules in the domain were not ordered.
Because the molecules in the fibrous structure and the uniaxial molecular assembly were ordered, LM-FFM images enabled us to characterize the surface oriented groups in these two structures. The frictional signals from the uniaxial molecular assembly were lower than those of the mica surface (0 mV) and almost the same as that of cadmium arachidate, which showed frictional signals of -50 mV, suggesting that the terminal methyl groups of the long alkyl chains of the uniaxial molecular assembly were directed toward the surface.21 The frictional signal of the fibrous structure surface was larger than that of the uniaxial molecular assembly in all directions and much larger than that of the mica surface along the perpendicular direction, showing the hydrophilic character of the surface of the fibrous structure. Substituents at the film surface deposited by a single withdrawal are generally hydrophobic, as in the case of the cadmium arachidate LB film.18,19 The hydrophilicity of the fibrous structure surface suggests molecular overturning and/ or bilayer formation upon the fibrous structure forming at the grain boundaries, caused by compression of the monolayer at the air/water interface. Given also the height of the fibrous structure, the fibrous structure can be reasonably assumed to have a bilayer structure. Figure 5 schematizes the uniaxial molecular assembly and the fibrous structure. Adsorption of Au Nanoparticles on Fibrous Structures. Molecule 2 has an ammonium moiety substituted at the alcohol moiety of 1. Since the fibrous structure of 1 had a hydrophilic surface, a fibrous structure with a positively charged surface will be obtained by adding molecule 2 to 1. Negatively charged
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Figure 6. π-A isotherm of (a) molecule 2, (b) mixture of 1:2 ) 60: 40, and (c) muxture of 1:2 ) 80:20.
substances such as Au nanoparticles can be adsorbed on the surface through electrostatic interactions.25,35,36 Figure 6 shows π-A isotherms of 2 and the mixture of 1 and 2 at the air-water interface. The π-A isotherms showed similar behavior to 1. The surface pressure started to increase at around A ) 1.3 nm2, and the monolayer was stable up to 40 mN/m. A shoulder was observed at 13 mN/m in the isotherm of 2. Upon further compression, a solid condensed phase appeared at 25 mN/m. By the addition of 1 to 2, the surface pressure at the shoulder decreased from 13 mN/m for pure 2 to 10 and 8 mN/m for mixing ratios of 1:2 ) 60:40 and 80:20, respectively. The shape of the π-A isotherm approached that of 1 with increasing ratio of 1 in the mixture. The isotherm for a mixing ratio of 80:20 was almost identical to that of 1. Figure 7 shows AFM images of an LB film of 2 and a 80:20 mixed LB film deposited at 35 mN/m on mica surfaces by a single withdrawal. A uniform surface structure was observed in the case of 2. Molecules 1 and 2 have the same structure except for the hydrophilic moiety at the end. Thus, a similar packing of molecules was expected at the air-water interface through intermolecular interaction at two amide moieties. Indeed, molecules 1 and 2 showed similar gelation ability. Table 1 summarizes the gelation properties of 1 and 2 in organic solvents and water. Molecules 1 and 2 formed organogels with nonpolar solvents such as toluene and hexane at room temperature. In chloroform, 1 and 2 formed organogels at low temperature. In water, 2 formed a hydrogel, while 1 could not be dispersed. The π-A isotherms indicated that the long alkyl chains determined the surface area, forming closely packed structures in the solid condensed phase. The difference in morphology between the LB films presumably resulted from the terminal hydrophilic group, namely, the alcohol versus ammonium moiety, suggesting that the assembled structures were determined through the intermolecular interactions between not only the amide groups but also the polar terminal groups. In general, ionic compounds interact with counterions in the subphase and form a stable monolayer at the air-water interface.9 Ionic interactions are isotropic and suppress the formation of anisotropic structures such as fibrous structures. The uniform film of 2 resulted from isotropic ionic interactions. Figure 7b shows the AFM image of an LB film of 1 and 2 mixed at a ratio of 80:20 (that of 60:40 is shown in Figure S1, Supporting Information). Fibrous structures similar to those of the LB film of 1 were observed. This result was consistent with that of the π-A isotherm of 80:20, which was almost identical to that of 1. The fibrous structure in the mixed LB film had the same height as in the case of 1 and was oriented in C6 symmetry. In addition to the fibrous structures, another type of structure was observed. The height and frictional images (Figure S3, Supporting Information), which were indicative of a hydrophobic surface, suggested that this structure was identical to the uniaxial molecular assembly observed in the film of 1.
Figure 7. AFM images of (a) LB film of 2 deposited at 35 mN/m and (b) film of 1 and 2 mixed at a ratio of 1:2 ) 80:20.
TABLE 1: Gelation Properties of 1 and 2 in Solvents during the Cooling Processa 1 2
hexane
toluene
CHCl3b
H2O
G G
G G
G G
I G
a At room temperature for 55 mM solution except for CHCl3. G ) gel; I ) insoluble. b 0.3 M at low temperature.
The surface-oriented groups of these fibrous structures of the mixed LB film were shown to be hydrophilic, that is, ammonium and alcohol moieties, by LM-FFM measurements (see Supporting Information). The positively charged ammonium moiety was expected to interact with the Au nanoparticles, which were negatively charged. Figure 8 shows the SEM image of the LB film of 1:2 ) 80:20 deposited on the mica substrate at 35 mN/m after the treatment with Au nanoparticle suspension. Au nanoparticles were arrayed one-dimensionally in C6 crystal symmetry. The negatively charged Au nanoparticles were adsorbed on the positively charged fibrous structures through electrostatic interaction. In the case of pure 1, the Au nanoparticles were not ordered (Figure S4, Supporting Information) owing to the nonspecificity of the interactions between the Au nanoparticles and 1. Figure 9 shows AFM images of a mixed LB film at the same position before and after casting of the Au
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Figure 8. SEM image of the LB film of 1:2 ) 80:20 deposited at 35 mN/m on a mica substrate after casting of Au nanoparticles. Scale bar ) 200 nm.
Organogelator 1 was fabricated in one-dimensional fibrous structures on mica surfaces by the LB method. The fibrous structures had typical dimensions of 1.2 nm (height) × 50100 nm (width) × 5-10 µm (length). On the mica substrate, two types of other structures were observed, namely, uniaxial molecular assemblies and uniform domains. LM-FFM measurements revealed that the fibrous structures and the uniaxial molecular assemblies had high crystallinity. The surface groups of the fibrous structures were hydrophilic, while those of the uniaxial molecular assemblies were presumably methyl groups. Organogelator 2 has an ammonium moiety instead of the alcohol moiety of 1, and it formed a uniform LB film. This difference in morphology may have resulted from the ionic interactions of the ammonium moieties. By mixing of molecule 2 with 1, a fibrous structure with positively charged surface was obtained. During further treatment with Au nanoparticles, the fibrous structure acted as a one-dimensional template for the Au nanoparticles. Acknowledgment. This work was partly supported by a Grant-in-Aid for Science Research from the Ministry of Education, Science, Sports and Culture of Japan. Supporting Information Available: Synthesis of 1 and 2 and figures showing AFM images of LB film of 1 on (3-aminopropyl)trimethoxysilane-coated silicon substrate (Figure S1), cast film (Figure S2), and mixed LB film (1:2 ) 60:40) deposited at 35 mN/m (Figure S3); LM-FFM images of monolayer LB films on mica substrates 1 transferred at 25 mN/m (Figire S4) and mixture of 1:2 ) 80:20 deposited at 35 mN/m (Figure S5); and AFM images of 1 deposited at 35 mN/m after casting of Au nanoparticles on LB films (Figure S6). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes
Figure 9. AFM images of mixed LB films. Measurements were made at the same position (a) before and (b) after the treatment with Au nanoparticles. The scale bar in panel a is 4 µm. The height profiles correspond to the white dotted lines.
nanoparticle suspension. There were no morphological differences between these AFM images expect for the height, which changed from 1 to 14 nm at the fibrous structure surface, showing that the casting of Au nanoparticles did not affect the fibrous structures.
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